(function() {
  'use strict';

  var has = Object.prototype.hasOwnProperty,
    prefix = '~';

  /**
   * Constructor to create a storage for our `EE` objects.
   * An `Events` instance is a plain object whose properties are event names.
   *
   * @constructor
   * @api private
   */
  function Events() {}

  //
  // We try to not inherit from `Object.prototype`. In some engines creating an
  // instance in this way is faster than calling `Object.create(null)` directly.
  // If `Object.create(null)` is not supported we prefix the event names with a
  // character to make sure that the built-in object properties are not
  // overridden or used as an attack vector.
  //
  if (Object.create) {
    Events.prototype = Object.create(null);

    //
    // This hack is needed because the `__proto__` property is still inherited in
    // some old browsers like Android 4, iPhone 5.1, Opera 11 and Safari 5.
    //
    if (!new Events().__proto__) prefix = false;
  }

  /**
   * Representation of a single event listener.
   *
   * @param {Function} fn The listener function.
   * @param {Mixed} context The context to invoke the listener with.
   * @param {Boolean} [once=false] Specify if the listener is a one-time listener.
   * @constructor
   * @api private
   */
  function EE(fn, context, once) {
    this.fn = fn;
    this.context = context;
    this.once = once || false;
  }

  /**
   * Minimal `EventEmitter` interface that is molded against the Node.js
   * `EventEmitter` interface.
   *
   * @constructor
   * @api public
   */
  function EventEmitter() {
    this._events = new Events();
    this._eventsCount = 0;
  }

  /**
   * Return an array listing the events for which the emitter has registered
   * listeners.
   *
   * @returns {Array}
   * @api public
   */
  EventEmitter.prototype.eventNames = function eventNames() {
    var names = [],
      events,
      name;

    if (this._eventsCount === 0) return names;

    for (name in (events = this._events)) {
      if (has.call(events, name)) names.push(prefix ? name.slice(1) : name);
    }

    if (Object.getOwnPropertySymbols) {
      return names.concat(Object.getOwnPropertySymbols(events));
    }

    return names;
  };

  /**
   * Return the listeners registered for a given event.
   *
   * @param {String|Symbol} event The event name.
   * @param {Boolean} exists Only check if there are listeners.
   * @returns {Array|Boolean}
   * @api public
   */
  EventEmitter.prototype.listeners = function listeners(event, exists) {
    var evt = prefix ? prefix + event : event,
      available = this._events[evt];

    if (exists) return !!available;
    if (!available) return [];
    if (available.fn) return [available.fn];

    for (var i = 0, l = available.length, ee = new Array(l); i < l; i++) {
      ee[i] = available[i].fn;
    }

    return ee;
  };

  /**
   * Calls each of the listeners registered for a given event.
   *
   * @param {String|Symbol} event The event name.
   * @returns {Boolean} `true` if the event had listeners, else `false`.
   * @api public
   */
  EventEmitter.prototype.emit = function emit(event, a1, a2, a3, a4, a5) {
    var evt = prefix ? prefix + event : event;

    if (!this._events[evt]) return false;

    var listeners = this._events[evt],
      len = arguments.length,
      args,
      i;

    if (listeners.fn) {
      if (listeners.once)
        this.removeListener(event, listeners.fn, undefined, true);

      switch (len) {
        case 1:
          return listeners.fn.call(listeners.context), true;
        case 2:
          return listeners.fn.call(listeners.context, a1), true;
        case 3:
          return listeners.fn.call(listeners.context, a1, a2), true;
        case 4:
          return listeners.fn.call(listeners.context, a1, a2, a3), true;
        case 5:
          return listeners.fn.call(listeners.context, a1, a2, a3, a4), true;
        case 6:
          return listeners.fn.call(listeners.context, a1, a2, a3, a4, a5), true;
      }

      for (i = 1, args = new Array(len - 1); i < len; i++) {
        args[i - 1] = arguments[i];
      }

      listeners.fn.apply(listeners.context, args);
    } else {
      var length = listeners.length,
        j;

      for (i = 0; i < length; i++) {
        if (listeners[i].once)
          this.removeListener(event, listeners[i].fn, undefined, true);

        switch (len) {
          case 1:
            listeners[i].fn.call(listeners[i].context);
            break;
          case 2:
            listeners[i].fn.call(listeners[i].context, a1);
            break;
          case 3:
            listeners[i].fn.call(listeners[i].context, a1, a2);
            break;
          case 4:
            listeners[i].fn.call(listeners[i].context, a1, a2, a3);
            break;
          default:
            if (!args)
              for (j = 1, args = new Array(len - 1); j < len; j++) {
                args[j - 1] = arguments[j];
              }

            listeners[i].fn.apply(listeners[i].context, args);
        }
      }
    }

    return true;
  };

  /**
   * Add a listener for a given event.
   *
   * @param {String|Symbol} event The event name.
   * @param {Function} fn The listener function.
   * @param {Mixed} [context=this] The context to invoke the listener with.
   * @returns {EventEmitter} `this`.
   * @api public
   */
  EventEmitter.prototype.on = function on(event, fn, context) {
    var listener = new EE(fn, context || this),
      evt = prefix ? prefix + event : event;

    if (!this._events[evt]) (this._events[evt] = listener), this._eventsCount++;
    else if (!this._events[evt].fn) this._events[evt].push(listener);
    else this._events[evt] = [this._events[evt], listener];

    return this;
  };

  /**
   * Add a one-time listener for a given event.
   *
   * @param {String|Symbol} event The event name.
   * @param {Function} fn The listener function.
   * @param {Mixed} [context=this] The context to invoke the listener with.
   * @returns {EventEmitter} `this`.
   * @api public
   */
  EventEmitter.prototype.once = function once(event, fn, context) {
    var listener = new EE(fn, context || this, true),
      evt = prefix ? prefix + event : event;

    if (!this._events[evt]) (this._events[evt] = listener), this._eventsCount++;
    else if (!this._events[evt].fn) this._events[evt].push(listener);
    else this._events[evt] = [this._events[evt], listener];

    return this;
  };

  /**
   * Remove the listeners of a given event.
   *
   * @param {String|Symbol} event The event name.
   * @param {Function} fn Only remove the listeners that match this function.
   * @param {Mixed} context Only remove the listeners that have this context.
   * @param {Boolean} once Only remove one-time listeners.
   * @returns {EventEmitter} `this`.
   * @api public
   */
  EventEmitter.prototype.removeListener = function removeListener(
    event,
    fn,
    context,
    once
  ) {
    var evt = prefix ? prefix + event : event;

    if (!this._events[evt]) return this;
    if (!fn) {
      if (--this._eventsCount === 0) this._events = new Events();
      else delete this._events[evt];
      return this;
    }

    var listeners = this._events[evt];

    if (listeners.fn) {
      if (
        listeners.fn === fn &&
        (!once || listeners.once) &&
        (!context || listeners.context === context)
      ) {
        if (--this._eventsCount === 0) this._events = new Events();
        else delete this._events[evt];
      }
    } else {
      for (var i = 0, events = [], length = listeners.length; i < length; i++) {
        if (
          listeners[i].fn !== fn ||
          (once && !listeners[i].once) ||
          (context && listeners[i].context !== context)
        ) {
          events.push(listeners[i]);
        }
      }

      //
      // Reset the array, or remove it completely if we have no more listeners.
      //
      if (events.length)
        this._events[evt] = events.length === 1 ? events[0] : events;
      else if (--this._eventsCount === 0) this._events = new Events();
      else delete this._events[evt];
    }

    return this;
  };

  /**
   * Remove all listeners, or those of the specified event.
   *
   * @param {String|Symbol} [event] The event name.
   * @returns {EventEmitter} `this`.
   * @api public
   */
  EventEmitter.prototype.removeAllListeners = function removeAllListeners(
    event
  ) {
    var evt;

    if (event) {
      evt = prefix ? prefix + event : event;
      if (this._events[evt]) {
        if (--this._eventsCount === 0) this._events = new Events();
        else delete this._events[evt];
      }
    } else {
      this._events = new Events();
      this._eventsCount = 0;
    }

    return this;
  };

  //
  // Alias methods names because people roll like that.
  //
  EventEmitter.prototype.off = EventEmitter.prototype.removeListener;
  EventEmitter.prototype.addListener = EventEmitter.prototype.on;

  //
  // This function doesn't apply anymore.
  //
  EventEmitter.prototype.setMaxListeners = function setMaxListeners() {
    return this;
  };

  //
  // Expose the prefix.
  //
  EventEmitter.prefixed = prefix;

  //
  // Allow `EventEmitter` to be imported as module namespace.
  //
  EventEmitter.EventEmitter = EventEmitter;

  //
  // Expose the module.
  //
  if ('undefined' !== typeof module) {
    module.exports = EventEmitter;
  }

  window.EventEmitter = EventEmitter;
})();
